Subject: Re: [PATCH bpf v5 1/3] bpf: don't return EINVAL from {get,set}sockopt when optlen > PAGE_SIZE

From: [email-masked]
Date: Wed, 17 Jun 2020 10:45:08 -0700

On 06/17, Alexei Starovoitov wrote:
> On Tue, Jun 16, 2020 at 06:04:14PM -0700, Stanislav Fomichev wrote:
> > Attaching to these hooks can break iptables because its optval is
> > usually quite big, or at least bigger than the current PAGE_SIZE limit.
> > David also mentioned some SCTP options can be big (around 256k).
> >
> > For such optvals we expose only the first PAGE_SIZE bytes to
> > the BPF program. BPF program has two options:
> > 1. Set ctx->optlen to 0 to indicate that the BPF's optval
> >    should be ignored and the kernel should use original userspace
> >    value.
> > 2. Set ctx->optlen to something that's smaller than the PAGE_SIZE.
> >
> > v5:
> > * use ctx->optlen == 0 with trimmed buffer (Alexei Starovoitov)
> > * update the docs accordingly
> >
> > v4:
> > * use temporary buffer to avoid optval == optval_end == NULL;
> >   this removes the corner case in the verifier that might assume
> >   non-zero PTR_TO_PACKET/PTR_TO_PACKET_END.
> >
> > v3:
> > * don't increase the limit, bypass the argument
> >
> > v2:
> > * proper comments formatting (Jakub Kicinski)
> >
> > Fixes: 0d01da6afc54 ("bpf: implement getsockopt and setsockopt hooks")
> > Cc: David Laight <[email-masked]>
> > Signed-off-by: Stanislav Fomichev <[email-masked]>
> > ---
> >  kernel/bpf/cgroup.c | 53 ++++++++++++++++++++++++++++-----------------
> >  1 file changed, 33 insertions(+), 20 deletions(-)
> >
> > diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> > index 4d76f16524cc..ac53102e244a 100644
> > --- a/kernel/bpf/cgroup.c
> > +++ b/kernel/bpf/cgroup.c
> > @@ -1276,16 +1276,23 @@ static bool  
> __cgroup_bpf_prog_array_is_empty(struct cgroup *cgrp,
> >
> >  static int sockopt_alloc_buf(struct bpf_sockopt_kern *ctx, int  
> max_optlen)
> >  {
> > -	if (unlikely(max_optlen > PAGE_SIZE) || max_optlen < 0)
> > +	if (unlikely(max_optlen < 0))
> >  		return -EINVAL;
> >
> > +	if (unlikely(max_optlen > PAGE_SIZE)) {
> > +		/* We don't expose optvals that are greater than PAGE_SIZE
> > +		 * to the BPF program.
> > +		 */
> > +		max_optlen = PAGE_SIZE;
> > +	}
> > +
> >  	ctx->optval = kzalloc(max_optlen, GFP_USER);
> >  	if (!ctx->optval)
> >  		return -ENOMEM;
> >
> >  	ctx->optval_end = ctx->optval + max_optlen;
> >
> > -	return 0;
> > +	return max_optlen;
> >  }
> >
> >  static void sockopt_free_buf(struct bpf_sockopt_kern *ctx)
> > @@ -1319,13 +1326,13 @@ int __cgroup_bpf_run_filter_setsockopt(struct  
> sock *sk, int *level,
> >  	 */
> >  	max_optlen = max_t(int, 16, *optlen);
> >
> > -	ret = sockopt_alloc_buf(&ctx, max_optlen);
> > -	if (ret)
> > -		return ret;
> > +	max_optlen = sockopt_alloc_buf(&ctx, max_optlen);
> > +	if (max_optlen < 0)
> > +		return max_optlen;
> >
> >  	ctx.optlen = *optlen;
> >
> > -	if (copy_from_user(ctx.optval, optval, *optlen) != 0) {
> > +	if (copy_from_user(ctx.optval, optval, min(*optlen, max_optlen)) !=  
> 0) {
> >  		ret = -EFAULT;
> >  		goto out;
> >  	}
> > @@ -1353,8 +1360,14 @@ int __cgroup_bpf_run_filter_setsockopt(struct  
> sock *sk, int *level,
> >  		/* export any potential modifications */
> >  		*level = ctx.level;
> >  		*optname = ctx.optname;
> > -		*optlen = ctx.optlen;
> > -		*kernel_optval = ctx.optval;
> > +
> > +		/* optlen == 0 from BPF indicates that we should
> > +		 * use original userspace data.
> > +		 */
> > +		if (ctx.optlen != 0) {
> > +			*optlen = ctx.optlen;

> I think it should be:
> *optlen = min(ctx.optlen, max_optlen);
We do have the following (existing) check above:
	} else if (ctx.optlen > max_optlen || ctx.optlen < -1) {
		/* optlen is out of bounds */
		ret = -EFAULT;
	} else {

So we shouldn't need any min here? Or am I missing something?

> Otherwise when bpf prog doesn't adjust ctx.oplen the kernel will see
> 4k only in kernel_optval whereas optlen will be > 4k.
> I suspect iptables sockopt should have crashed at this point.
> How did you test it?
The selftests that I've attached in the series. The test is passing
two pages and for IP_TOS we bypass the value via optlen=0 and
for IP_FREEBIND we trim the buffer to 1 byte. I think this should
cover this check here.

One thing I didn't really test is getsockopt when the kernel
returns really large buffer (iptables). Right now, the test
gets 4 bytes (trimmed) from the kernel. I think that's the only
place that I didn't properly test. I wonder whether I should
do a real iptables-like setsockopt/getsockopt :-/